Dec 25, 2013 11:07 PM

I was just wondering, can bad weather damage fossils in a short space of time? I was wondering because when there has been bad snow, there is loads of fossils, but they are always very fragile and it's hard to get them out of the ground without damaging them, but in dry conditions, they are really strong fossils, which are much harder than they are then when they have been in bad weather conditions.

Is it just a coincidence or can bad weather damage fossils in a few hours, or do things like erosion take years?

With the fragile fossils you mention, they are often in friable clays or siltstones, which may have micro-cracks. When we try to remove a fossil, it will often break (or already be broken) along one of those micro-cracks. ...Or the fossil may be sufficiently weak that it will break almost anywhere.

During rains, erosion will gradually remove particles and in doing so, fossils will become exposed or more-exposed. If such a fossil is already broken (by micro-cracks) or very weak, it may break simply through removal of the particles of rock that were supporting it.

During summer drought, the clays/siltstones may shrink significantly. That can exaccerbate the cracking - breaking fossils in-place and allowing rainwater to penetrate deeper next time it rains. That is, there can be damaging shrink/swell cycles in the rock.

During freezing conditions, if the rock is wet, the ice will expand in the micro-cracks and prise little chunks of the rock apart - perhaps breaking fossils in the process, the pieces later falling away when the ice melts.

Repeated freezing-thawing cycles can have a significant erosional effect, even on much harder rocks. In such cyclic cases, the ice wedges force cracks to open wider and wider, which can result in even large blocks of rock becoming loose. In mountains above the tree line, this sort of erosion is responsible for the jagged appearance of the rocks as a whole (in combination with other factors).

Fossils can be harder when they are dry either because the rock supporting them is harder, or because they are strengthened by a temporary cement. Let me explain that last bit... In some rocks, the cement holding the particles together is rather soluble in water. When the rocks are subjected to cycles of wet and dry, water soaks in and dissolves the cement as it goes, then as it dries, the fluid migrates to the surface, where the dissolved cement may partly re-precipitate. That results in two things: a) the rock perhaps including fossils becoming harder (than when wet) at the surface; b) the slightly deeper rock becoming weaker because the cement is gradually removed. This whole process can result in hollows forming behind the surface, giving a strange honeycombed effect. If the rock contained veins of different, less-soluble, minerals, they are likely to remain, forming at least some of the walls of the honeycomb.You can see that on some coastal cliffs.

Noticed this Eleganticeras elegantulum in half a cannon ball has been got at by the weather, it has been on the rockary for a while but now all of the center has gone and the outer whorle has dropped off, too much to restore.

You are right Dan, erosion of a rock sometimes exposes the fossils within, but unfortunately you have to find the rock with the fossil exposed before the fossil is damaged.

Especialy in a beach environment like the Holderness coast.

If the fossil is harder than the rock matrix around it you will usually find the fossil is stood proud of the matrix, that is a good sign because the rock will be easier to remove from the harder object/fossil as the matrix will be softer.

If the fossil is worn to the surface of the matrix of the rock with the fossil inside it will be smooth and hard to prep because of the fossil being of a similar hardness or softer than the surounding rock.

One of my son's once found a pyritic cannonball coming out of the Boulder Clay Cliff on the Holderness at Tunstall, as he picked it out of the mud it fell in half revieling an ammonite in the dead center of the nodule.

Looked like the weather and sea water had got to it, splitting it in half.